Loop copying over vertex coordinates is not working
List comprehension

Not sure why you are getting the issue. For Mesh.vertices make sure you are running code in object mode.  

Question code can IMO be simplified to 

import bpy
context = bpy.context

ob = context.object
mw = ob.matrix_world
me = ob.data

vert_list = [list(mw @ v.co) for v in me.vertices] # 2.80
#vert_list = [(mw @ v.co)[:] for v in me.vertices] # 2.80 alternate

#vert_list = [list(mw * v.co) for v in me.vertices] # 2.7x

print(vert_list)


in this case see no need to predefine vert_list or enumerate to index.

Alternatively using bmesh in edit mode

import bpy
import bmesh

context = bpy.context

ob = context.edit_object
mw = ob.matrix_world
me = ob.data

bm = bmesh.from_edit_mesh(me)
bmesh.ops.transform(bm,
        verts=bm.verts,
        matrix=mw
        )
vert_list = [v.co[:] for v in bm.verts]
print(vert_list)